N.Y. County Law § 299-U
Where two or more adjoining counties have established agencies and have created watershed districts for the purposes of this act in the watershed common to the counties, the agencies and the administrative heads or bodies, subject to the approval of their respective boards of supervisors, may enter into agreements for the planning, installation, construction, operation, maintenance, assessments of costs and repair of a joint small watershed project. Any agency or agencies within this state, established by a county or counties for the purposes of this act with respect to small watersheds, may, subject to the approval of the board or boards of supervisors of the county or counties, enter into agreements respecting joint projects with one or more "local organizations" in other states as defined in section ten hundred two of title sixteen of the United States code. The provisions of all sections of this article concerning the planning, installation, construction, operation, maintenance, assessments of costs and repair of, and the approvals required for a project within a single county of this state, shall apply with equal and full force and effect to a joint project, but nothing herein shall be deemed to prevent or restrict the additional applicability thereto of any requirements of another state in any matter relating to its interest or the interests of its subdivisions therein.
N.Y. County Law § 299-U